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Magnesium is light, abundant and it can store up to 7.6 wt. % of
hydrogen forming the hydride MgH2 and therefore it is a promising material for
hydrogen storage. However, the H-sorption occurs at relatively high
temperatures with slow kinetics. Beside this, Mg and MgH2 surfaces are highly
reactive, easily forming oxide or hydroxide layers that lower the level of storage
properties. Mg-based nanocomposites have been studied in the last few years
to overcome these limitations. The grain size reduction of Mg or MgH2 to the
nanometric scale and the addition of catalysts as transition metals or its
hydrides can promote fast kinetics at lower temperatures. The formation of a
fluorinated layer on Mg surface enhances its stability in the sorption cycles
avoiding the usual contamination with oxygen. Reactive milling under H2
atmosphere is one of the processing routes that has been recently investigated
for the preparation of Mg-based nanocomposites, and promising results have
been obtained. In the present work, the effects of different nanocrystalline
additives (MgF2, Fe, NbH0,89, FeF3, VF3) into Mg processed by reactive milling
were studied. The aspects analysed in this work were the influence of the
additives in MgH2 synthesis during milling and in the desorption behavior. A
combined catalytic effect was observed due to the MgF2 and Fe (or NbH0,89)
action in MgH2 synthesis during processing. The transition metal fluorides also
promote MgH2 synthesis. A fluorine transfer reaction occurs from the fluoride to
Mg, generating MgF2 and transition metal (or its hydride) nanoparticles in the
mixture. An important catalytic effect of Fe during H-desorption of MgH2 was
also observed.